This class module binds one or more subroutines of your devising to a Perl variable. All variables can have FETCH, STORE and DESTROY callbacks. Additionally, arrays can define CLEAR, DELETE, EXISTS, EXTEND, FETCHSIZE, POP, PUSH, SHIFT, SPLICE, STORESIZE and UNSHIFT callbacks, and hashes can define CLEAR, DELETE, EXISTS, FIRSTKEY and NEXTKEY callbacks. If these term are unfamiliar to you, I really suggest you read perltie.

With Tie::Watch you can:

. alter a variable's value
. prevent a variable's value from being changed
. invoke a Perl/Tk callback when a variable changes
. trace references to a variable

Callback format is patterned after the Perl/Tk scheme: supply either a code reference, or, supply an array reference and pass the callback code reference in the first element of the array, followed by callback arguments. (See examples in the Synopsis, above.)

Tie::Watch provides default callbacks for any that you fail to specify. Other than negatively impacting performance, they perform the standard action that you'd expect, so the variable behaves "normally". Once you override a default callback, perhaps to insert debug code like print statements, your callback normally finishes by calling the underlying (overridden) method. But you don't have to!

To map a tied method name to a default callback name simply lowercase the tied method name and uppercase its first character. So FETCH becomes Fetch, NEXTKEY becomes Nextkey, etcetera.

Here are two callbacks for a scalar. The FETCH (read) callback does nothing other than illustrate the fact that it returns the value to assign the variable. The STORE (write) callback uppercases the variable and returns it. In all cases the callback must return the correct read or write value - typically, it does this by invoking the underlying method.

-shadow (default 1) is 0 to disable array and hash shadowing. To prevent infinite recursion Tie::Watch maintains parallel variables for arrays and hashes. When the watchpoint is created the parallel shadow variable is initialized with the watched variable's contents, and when the watchpoint is deleted the shadow variable is copied to the original variable. Thus, changes made during the watch process are not lost. Shadowing is on my default. If you disable shadowing any changes made to an array or hash are lost when the watchpoint is deleted.

For array and hash Watch objects, the -value key is replaced with a -ptr key which is a reference to the parallel array or hash. Additionally, for an array or hash, there are key/value pairs for all the variable specific callbacks.